1. Field of the Invention
The present invention relates to a screw rotor including a resin rotor formed around a metallic shaft.
2. Description of the Related Art
In order to strongly fix the shaft to the rotor in a screw rotor including a resin rotor formed around a metallic shaft, Japanese Patent Laid-Open No. Hei6-123292 describes that spiral grooves are formed in the shaft. However, the groove formed in the shaft creates a difference in level on an inner surface of the rotor. Therefore, there is a problem that stress is concentrated on edge parts thereof and hence cracks are generated.
Japanese Patent No. 3701378 describes a screw rotor in which grooves having a cross section in a circular arc shape are formed in a shaft and adjacent grooves are connected by a non-angular and smooth mountainous shape curve.
By the shaft shape of Japanese Patent No. 3701378, it is possible to ease the concentration of stress on an inner surface of the rotor. However, it is not possible to form such a non-angular groove by a normal working machine such as a screwing machine and a multiple lathe. Therefore, there is a problem that a finish processing requires a manual work with taking a lot of time and cost.
Since the shaft shape of Japanese Patent No. 3701378 requires a back clearance for tools when the spiral grooves are processed, a diameter on both sides of a part in which the spiral grooves are formed is narrow. Therefore, there is a difference in level in the rotor at the part in which the diameter of the shaft is narrow, and hence there is a case where the stress in the axial direction at the time of forming the rotor and driving causes cracks in the rotor.
Further, a depth of the spiral grooves in the shaft shape of Japanese Patent No. 3701378 is described to have about 1% of a shaft diameter. For example, however, in a shaft having a diameter of 40 to 80 mm, a depth of the grooves is shallow with 0.4 to 0.8 mm. There is a problem that the spiral groove is worn away soon due to rotary torque at the time of driving, loads in the thrust direction and the radial direction, and shear stress caused by a difference in thermal expansion rate between the shaft and the rotor.